1. Field of the Invention
The present invention relates to a polarization separating element for synthesizing or separating beams into two beams of which the polarized directions are perpendicular to each other and a method of manufacturing the same, and particularly to a polarization separating element used in an optical low pass filter and the like for removing the moiré fringe in a video optical system and a method of manufacturing the same.
2. Description of the Related Art
In video optical systems having the structure that light receiving portions are periodically arranged as in a CCD (charge coupled device) camera or a CMOS (complementary metal-oxide semiconductor) image sensor, the image deterioration phenomenon called moiré may be generated. In order to prevent this phenomenon, a polarization separating element for synthesizing or separating beams into two beams of which the polarized directions are perpendicular to each other is provided.
Generally, crystals such as quartz or the like are used as a conventional polarization separating element. In this case, the light can be separated by inclining the crystal axis with respect to the incident light, and thus the polarization separating element depends on the birefringence of the crystal material itself. This is the polarization separating element using the birefringence of the material.
In addition, the polarization separating element has birefringence by regularly arranging particles having the wavelength smaller than that of the light. This is called structural birefringence (for example, see Japanese Examined Patent Application Publication No. 7-66084). For example, as shown in FIG. 11, dielectric thin films 101 and 102 each having a different refraction index are alternately laminated to form a structural birefringence plate 100. In the structural birefringence plate 100, the beams R incident into the inclined section 100a of the lamination surface of the lamination body formed by alternately laminating the dielectric thin films 101 and 102 are separated into ordinary light Eo and extraordinary light Ee1, of which the polarized waves are perpendicular to each other. The ordinary light Eo is refracted in the same direction as the incident direction and the extraordinary light Ee1 is refracted in the direction of an angle between the direction of the incident beam R and a lamination surface 100c and an angle determined by the shape birefringence value, thereby, separating the light. Thus, the structural birefringence plate functions as the polarization separating element.
However, the quartz is generally used as the polarization separating element using the birefringence of the material, but there are problems in that the quartz is expensive and it is difficult to perform the processes such as cutting or polishing with respect to the quartz.
Further, with respect to the structural birefringence body manufactured by laminating multi-dielectric thin films by a sputter device or the like, the laminated thin film is cut to be inclined and is used. However, there are problems in that the film thickness corresponding to the used area is required, (for example, in the case of manufacturing the structural birefringence body having several millimeters of square, it is necessary that at least 1000 dielectric thin films of several tens of millimeters be laminated), the manufacturing is difficult because of the removal due to the stress and the manufacturing time becomes long.
In addition, even in any case, in order to separate the light in the plural directions, a plurality of the structural birefringence plates need be laminated and thus there is a problem in that the thickness of the element increases, as shown in FIG. 11. In addition, in FIG. 11, a reference numeral 200 indicates the other structural birefringence plate formed by alternately laminating dielectric thin films 201 and 202 each having a different refractive index. The ordinary light Eo emitted from the structural birefringence plate 100 is incident into the inclined section of the laminated surface of the laminated body of the structural birefringence plate 200 and is separated into the ordinary light Eo and the extraordinary light Ee2 of which the polarized waves are perpendicular to each other.